Process of and apparatus for the stabilization of high-frequency gas and glow discharges



B. BERGHAUS RAT April 27, 1965 PROCESS OF AND APPA US FOR THESTABILIZATION OF HIGH-FREQUENCY GAS AND GLOW DISCHARGES Filed Aug. 17,1960 I N VEN TOR erzzaf 50g/mu LUM EER uw ATTORNEY@ United States PatentO 3,181,029 PROCESS 0F AND APPARA'EUS FR THE S'EAEEEU- ZA'HN @illtllGH-EREQUENCY GAS ANB @MEW DSCl-HARGES Bernhard Berghaus, Zurich,Switserlaud., assigner to Elchtrophysilcalische Anstalt BernhardBerghaus, Vaduz, Liechtenstein, a corporation of Liechtenstein FiledAug. 17, 1960, Ser. No. 59,276 Claims priority, application Switzerland,Aug. 17, i959, 7 7 ,044/ 59 2 Claims. (sill. S15-111) This inventionrelates to the initiation, maintenance aud operation of high-frequencygas and glow discharges in a discharge vessel, Vparticularly to aprocess of and an apparatus for the stabilization of such discharges.

During the operation of high-frequency gas and glow discharges,particularly during the initial time after the current for suchdischarges in a discharge vessel is switched on, it is known thatmeasures must be provided in order to reduce possibilities for a glowdischarge turning into an arc discharge between the parts to which avoltage is applied, as such arc discharges or spark discharges usuallycause more or less `serious combustion phenomena on the surfaces of therespective parts to which a voltage is applied. The tendency of a glowdischarge to a transition into an arc'discharge is particularly strongduring the so-called starting period ofthe discharge process, thusduring theinitial time following the closing of the circuit of suchadischarge section. Especially in the caseof metallurgical processeswhich are carried out by means of a gas or glow discharge and for whicha relatively high operating temperature must be maintained, thesetendencies towards such undesirable arc discharges are observed quitefrequently. impurities on the res ective work pieces or stronglyemitting spots thereon, or enclosed gas remnants suddenly bursting ontfrom metal surfaces or liquefied material, andthe dire, are chief causesfavoring the transition `from a glow discharge into an arc discharge.After a certain operation time at a prescribed temperature for therespective process, the tendencies towards such undesirable phenomenaare known to become considerably less strong, and a rather steadyoperating condition is obtained on the discharge path. However, evenwith normal operation, safety measures must be provided to eliminate anydetrimental action of such possible arc discharges in the case of asudden outburst of gas from the heated surfaces or in the case of amomentary tendency towards a transition into an arc discharge which mayappear from any other reasons.

Various suggestions have already been made for the protection againstthe detrimental action of such arc discharges which may occur during thestarting period and operation of high`frequ'ency glow discharge Asc-called starting process is for instance described in Patent No.2,884,511 which reduces already considerably the tendency of thedischarge towards a tnansition into an arc discharge. Moreover, theshort reduction of `the opera-ting voltage at the discharge vessel isdescribed in this patent application, this reduction being made in orderto limit the current, i.e. to suppress any flash-over about to appear insuch cases where an adjustable mayirnum current is exceeded or where anadjustable minimum voltage is underpassed. Also in Patent No. 2,884,511,dated April 28, 1959 suitable measures are described for switching oilthe current when hash-overs appear.

With the use of all the devices known to this day for switching off thecurrent when flash-overs occur, an arc flashover is supposed to takeplace first, if only for a short time, in order to put the switch deviceinto operation and to reduce the operating voltage at the dischargevessel which extinguishes the arc. But this means that the electric arcwhich should be eliminated altogether, exists at least for -a shorttime. Experience, however, has shown that if an arc appears, though onlyfor a short time, undesirable consequences are inevitable, Aforinstance, socalled burning spots appear kon the surface of polishedmetal parts.

It is accordingly an object of this invention to suppress an arc aboutto be formed early enough so; that the undesirable electric arc or sparkflash-over actually does not take place.

The process according to this invention for the stabilization ofhigh-frequency gas and glow discharges in a discharge vessel by reducingthe operating voltage at the moment of sudden `clramges of an operationquantity of the discharge is characterized by the fact that thenecessary time for reducing the voltage following the formation andtiashover of a spark or arc discharge in the discharge vessel is madeshorter than heretofore.

This invention relates moreover to a device for carrying out the processwith a switch member designed to reduce the operating Voltage, beingcontrolled by a sensing member which reacts to sudden changes of anoperation quantity characterized by the fact that the action time forboth the sensing member and the switch member is made shorter than thetime o-f build-up and flash-over for a spark or arc discharge in thedischarge vessel.

This invention is described hereafter in an embodiment illustrated byway of example in the hereto attached wiring diagram of an installationfor high-frequency electric glow discharges.

The simplified wiring diagram, which embodies in large part thestructure shown and described in Patent No. 2,884,511, represents aninstallation for carrying out the process and especially for such casewherein, by means of a glow discharge, the nitriding of three steeltubes 2a, 2b, 2c takes place. The tubes 2a, 2b, 2c are each combinedwith the connections 3a, 3b, 3c over an insulated current lead-in.inside of each tube and insulated therefrom, an internal lead isarranged consisting, for instance, of a 7 mm. wide steel wire. All theinternal leads in the receptacle have parallel connections and areconnected with the terminal 4E- over an insulated current lead-in.Connected to a pump (not shown) is an exhaust line 5 as well as a gaslead-in 6 with the control valves 45 and 46.

The current supply is, as an example, a parallelly connected three-phaseAC. current over a three-phase variable voltage transformer 7 and threerectifying tubes 15a, 15b, 15e which feed the terminals 3a, 3b, 3c sothat the tubes 2a, 2b, 2c act as cathodes against the internal lead. Thevariable voltage transformer 7 is shown only with its secondarywindings. The primary windings may be connected to the polyphase main bymeans of a star or delta-connection. The variable voltage transformer 7is provided with a mutual regulating mechanism 9 for all the three tapsat the windings, wherein, by means of the reversible motor il, it can beadjusted to higher or lower secondary voltages. The regulating mechanism9 is moreover provided with a scale i3 which, for the initial and endadjustment, is controlled by a contact which, if so desired, may bearranged for signalling intermediary adjustments.

The three secondary windings of the variable voltage transformer 7 areinterconnected to the connections 3a, 3b, 3c, respectively, over thecumulative grid rectifying tube 15a, 15b, 15e, respectively, which maybe disconnected independently one from the other over the controlmembers la, idly, 16o, respectively.

The secondary windings of the variable voltage transformer 7 areoperated in star-connection, however, an impedance coil 23a, 23]), 23C,respectively, is connected inthe intermediate line from the neutralconductor 22 to the three secondary windings. All the three impedanceing or sensing member 28 which is also connected to the three secondaryphase voltages over the three impedances 27. This checking or sensingmember 28 reacts to all voltage changes exceeding an adjustable minimumvalue between the neutral line 22 and the three phases, and that independance on their actual-sequence which will be explained hereinbelow.YThe sensing member 28 is con- -n'ected to a line each by the threecontrol members toa, lob, li6c.

Thewhole installation is controlled by a central control device 29 towhich a cadence tapper or timer 3d is connected. The controlmember 29 isconnected, via the control line 3l, to the supply unit 32 of theadjusting motor 1l of the variable voltage transformer 7 and receivesthe revertive communication of thel scale 1.3 over the line 33.

Moreover, the supply unit 33 for the adjusting motor 25 of the chokes 23is activated in one or the other direction over the control line 37. Y

The functional sequence of the control device 29 is provided withsuitable relays, step-by-step selectors or electronic means in such amanner that the necessary con- -trol steps follow one another, the nextstep being introduced, however, only`in the moment when the previousstep is signalled back to the control device 29 to have taken place. If,therefore, the just passing step is stopped by a checking memberindependent from the control device 29, the next step cannot take placeunless the stopped control step is freed and terminated by the checkingmember and the revertive signalY received by the control device 29. Sucha control member is represented by theV sensing member 28 which, whenbeing activated over the control device 29,V inuences in a suitablemanner the supply units 32 and 38 for the adjusting motors of thevariable voltage transformer 7.

The embodiment of this installation according to the described principaldiagram is kprovided for an all-automatic program control at the settinginto working order of the discharge vessel l as well as for theautomatic control of the tubes to be treated 2a, 2b, and 2c during thewhole treating time as regards the temperature, the latter being checkedon the one hand by an output meter i8 and on the other hand, attached tothe tube 2a, by a thermoelectric element 49 with the registering `andregulating device 50 which, for its part, influences the control device29. If the control device 29 is suitably designed, a predeterminedvoltage may be set to the rectier tubes 551, b, and 15e and also to theterminals 3a, 3b, and 3c by means of the regulating motor M and thevoltage taps 9 at the secondary windings of the transformer '7. It isunnecessary to describe herein in detail the functions of the controldevice '29 `as a similar installation is shown and explained in full inthe above-mentioned Patent No.

2,884,511, wherein the parts dit, 44 and 47 are shown as Y j influencedby a phase voltage changing at the same time with the frequency of thesupply voltage. On the other hand, the respective checking connection isinfluenced if the respective phase voltage changes considerably quickeras this is the case if at Vany spot of the tube connected to this phasea tendency for the formation of an are discharge arises, YThe threechecking connections provided in the sensing member 2S are connectedindividually with each of the three control members 16a, 15b, and idcwhich provide the grid control of the rectifier tubes 15a, 15b, and 15Cconnected to this phase. The design of the respective checkingconnections and the operation of the control devices for blocking therespective rectifier tubes is generally known in pulse controlengineering so that a detailed description is superiiuous.

It is, however, important for the present process that the actuatingtime required by the checking connections and the grid control devicesfor the blocking of the respective rectifier tube may be madepractically of any desired short duration. For carrying out the presentprocess, the whole actuating time required for the operation of thechecking members in the sensing device 28 and of the control members la,lob, and 16e, and the parent rectifier tubes 15a, 15b, and 15C, must bemade shorter than the production and hash-over time needed for a sparkor arc discharge to be formed in the discharge vesse if this rule isobserved, it will be possibie to suppress an incipient arc dischargealready during its forma tion so that, in any case, an arc or sparkhash-over between the tubes Za, 2b, and 2c, and the parent innerelectrodes can be suppressed.

rThe present process was made possible when, by physicai tests on themechanism of the transition from anelectric glow discharge into an arcdischarge, it was established that already before the arc dischargeactually takes place which is known to cause a reduction of theoperating voltage of the glow discharge to about 20 to 5() v., thebuild-up' of the arc takes place, though within a short but measurabletime. This time interval preceding the arc ash-over is already combinedwith a voltage reduction and may be established by suiciently sensitivesensing members in the current supply line to the respective dischargepath. These sensing members must be so designed as to make thedifference between these short-lived Y voltage reductions taking placevery rapidly and the usual Y voltage tiuctuations produced, forinstance, in connection with the frequency of the alternating voltagewith A.C.`

operation. But this is not very difficult to find out when operatingwith 50 cycles A.C. because the time interval necessary for theformation of an arc hash-over is much shorter than the duration of ahalf cycle lof an 'A.C.

voltage. f

After establishing by the sensing member an arc discharge about to beformed, care must be taken during the remaining time interval until the`actual iiash-over of the arc that such an arc does not actually .takeplace at all. Thus, with the described embodiment, the operating voltageat the respective tube is suddenly reduced strongly or disconnectedaltogether, for which purpose electronic connecting members workingpractically without moments of inertia must be used. This kind ofconnecting member is generally known today and may be operated withinmicroseconds. The duration of disconnection of the operating voltage maybe relatively short, since the respective are discharge has beeninterrupted already during the interval of its formation sothat there isno strong ionization and local concentration to be feared. It has beenshown that disconnections ranging within a few milliseconds Ymay besur'iicient to eliminate a new arc to be formed suddenly at the samespot after reconnection of the current.

Instead of the operating voltage being controlled at the three phaseshinted thereto in the described embodiment, the current power inthethree supply lines may also be checked. With rectifier operationasdescribed in this embodiment and as experience shows, alternatingvoltages deviating from the sinusoidal form are often to be observed atsuch spots to which the sensing connections are applied via theimpedances 27. The sensing connections mustthen be adjusted so as not tobe actuated in spite of such'distorted voltage curves being produced.The carrying kout of the present process will naturally be much easierif the respective discharge path arenoso is operated with direct currentof a constant Voltage. In this case, the respective sensing connectionmay be designed in such a manner that, practically, any shortlivedvoltage iluctuation will cause this connection to actuate, and a shortreduction of the voltage or disconnection of the voltage to be obtained.As a connecting member, a grid controlled tube or a controllablesemi-conductor diode with a respective eiciency may also be employed. Inview of the short time intervals at disposal'from the rst appearance ofsigns of a formation of an arc discharge to shortly before the actualarc flash-over takes place, mechanical or electro-mechanical connectingmembers cannot be used for this process. On the contrary, the shortvoltage reduction needed for i the suppression of the arc flash-over maynot only be 0btained by disconnecting the current supply but also by arespective short bridging of the connections at the discharge vessel, aprotective impedance being provided in the bridging current circuitwhich permits the elimination of an increase of the charge of thecurrent supply beyond an admissible top value during the bridginginterval. For such bridging, already known in arc and gas dischargeengineering, there exist special electronic connecting devices which arewell known and need no detailed description.

Special attention is drawn to the fact that the process as describedabove is not only suited for the treatment of work pieces in an electricglow discharge, as outlined in the embodiment, but it can also be usedwith advantage for the so-called jet discharges as described in detail,for instance, in Patent No. 2,884,511. The control of the current supplycircuit for establishing an incipient formation of an arc discharge maybe substituted by optic checking of the discharge ath between theelectrodes to which a voltage is applied, for instance, by means ofelectric photocells. It has been experienced that the incipientformation of an arc can be established by this means and be used for theoperation of respective connecting members.

I claim:

1. The method of stabilizing a high current glow discharge betweenelectrodes in a vessel and wherein operating voltage is impressed onsaid electrodes, comprising the steps of: sensing the commencement ofchanges in the character of said discharge, which changes presageformation of an arc; and, in response to said changes, reducing theoperating voitage on said electrodes, completing said voltage reducingstep in a time interval following said sensing step which is less thanthe time necessary for formation of an arc.

2. rihe method of stabiiizing a high current glow discharge betweenelectrodes in a vessel and wherein operating voltage is impressed onsaid electrodes, comprising the steps of: sensing the commencement ofchanges in the character of said discharge, which changes presageformation of an arc; and, in response to said changes, reducing theoperating voltage on said electrodes, completing said voltage reducingstep in a time interval of the order of a few milliseconds and to avalue below the ignition voltage for an arc discharge.

References Cited in the le of this patent UNITED STATES PATENTS

1. THE METHOD OF STABILIZING A HIGH CURRENT GLOW DISCHARGE BETWEENELECTRODES IN A VESSEL AND WHEREIN OPERATING VOLTAGE IS IMPRESSED ONSAID ELECTRODES, COMPRISING THE STEPS OF: SENSING THE COMMENCEMENT OFCHANGES IN THE CHARACTER OF SAID DISCHARGE, WHICH CHANGES PRESAGEFORMATION IN AN ARC; AND, IN RESPONSE TO SAID CHANGES REDUCING THEOPERATING VOLTAGE ON SAID ELECTRODES, COMPLETING SAID VOLTAGE REDUCINGSTEP IN A TIME INTERVAL FOLLOWING THE SENSING STEP WHICH IS LESS THANTHE TIME NECESSARY FOR FORMATION OF AN ARC.